Do you mean PostProcessing in general?
If so, you can only use Pure Gamma and have PostProcessing deactivated.
Post Processing brings some nice effects like Glare (which boosts lights and such). And sure PP will bring you better colors and esp auto exposure.
In Pure Gamma i feature an auto exposure like brightness adaption, but it is not as goos as like in PostProcessing.
The PP off mode in AC is really a hack to have AC running with much fps, BUT PostProcessing brings many things which the game makes much nicer! If its only for the Tonemapping, i would say PP is actually necessary.
Tonemapping is a very needed process to "see" the colors eye friendly!
So without PP and tonemapping, the HDR image (every modern game is rendered in HDR) is directly send to the GPU.
Thatswhy it is so fast. BUT, you can't manage the HDR tones to be correctly displayed in SDR (what your monitor can show)
So typically you play in SDR (8bit per color channel = 24bit) and those color values (tones) have a limited range (0 lowest and 255 highest possible value)
In HDR you have nearly no limit and like in AC the color of the Sun is like 100 times higher then the monitor can display. Also the bright parts in the world are over that SDR limit.
So you need a process like a compression to get this HDR tones into SDR. This is called tonemapping. And this will take care, that you can see all the colors without a clipping limit.
This is Without PP and tonemapping (tones are cut, color burn):
View attachment 1468301
This is with PP and tonemapping:
View attachment 1468302
And this is Pure LCS, where the tones are correctly rendered in linear colors space:
View attachment 1468306
So actually Pure LCS would be the way to go to display the correct colors. It is like the same like the HDR picture, but with a decent tonemapping and colors are not screwed by the AC shaders...